Collision induced dissociation (CID) is a widely-used technique for the controlled fragmentation of precursor ions in a quadrupole ion trap (QIT). CID is commonly performed by applying a dipolar oscillatory excitation voltage to opposite QIT electrodes, also referred to as supplementary excitation. When the excitation voltage has a frequency at or near an ion's frequency of motion, energy from this field will be absorbed by the ion, increasing the ion's kinetic energy. The increased kinetic energy is converted into internal energy via collisions with the buffer gas, which can cause the ion to dissociate.
As the ion is excited, the amplitude of its oscillatory motion grows larger. In a pure quadrupolar field with no buffer gas collisions, the ion amplitude would grow linearly with time, where the slope of this growth is determined by the magnitude of the resonant excitation field. In a pure quadrupolar field, the electric field, and thus the force on an ion, varies linearly with its position, as in Equation 1, below:
                              E          x                =                              -                                          Φ                0                                            r                0                2                                              ⁢          x                                    (        1        )            
where Ex is the electric field in the x direction, Φ0 is the voltage difference between opposite rods, and r0 is the field radius. However, all QITs incorporate some proportion of higher order non-linear field components due to the truncation of the hyperbolic surfaces, the adaptation of one or more electrodes with ejection apertures, and departures from ideal surface geometry and electrode spacing caused by manufacturing errors and tolerances. As an example, the electric field contribution from an octopolar field, for comparison, is given in Equation 2.
                              E          x                =                  -                                                    2                ⁢                                  Φ                  0                                                            r                0                4                                      ⁡                          [                                                x                  3                                -                                  3                  ⁢                                      xy                    2                                                              ]                                                          (        2        )            
In an octopolar field (or other higher order field), the force on an ion varies with position in a non-linear fashion. “Cross terms” also are to be found in these fields, where the force depends on the ion position in the y or z dimensions in addition to its position in the x dimension. The influence of higher order fields causes the amplitude growth of an ion's motion during excitation to be non-linear with time, and at large displacements the frequency of ion oscillation changes. Due to the resonant nature of the excitation process, the effect of the resonance excitation field is diminished as the ion frequency shifts away from the frequency of the excitation voltage. The ion may be subsequently returned to a resonance condition as the result of collisions with the buffer gas, which reduce the ion's amplitude of motion and cause the ions frequency to shift back to its original value. The amplitude of ion motion and the frequency of ion oscillations will fluctuate in a beating pattern as the ion comes into and out of resonance with the supplementary excitation field, as illustrated in FIG. 1.
The transfer of ion kinetic energy into ion internal energy via buffer gas collisions has been extensively modeled in the mass spectrometry literature, and the outcome of a collision has been shown to depend on the relative kinetic energy of the ion/neutral encounters, as well as the internal energy of the ion. When collisions occur with high relative kinetic energy and the ion has low internal energy, the ion internal energy will tend to increase. In contrast, when collisions have lower relative kinetic energy and the ion has high internal energy, the ion internal energy will tend to decrease. Therefore, when the ion shifts out of resonance with the supplementary excitation field and collisions occur, the ion kinetic energy is quickly lost, resulting in reduction of internal energy deposition in subsequent collisions. This phenomenon results in decreased ion fragmentation efficiency, thereby reducing the number of product ions formed in a given time and requiring longer times (relative to fragmentation in a hypothetical pure quadrupolar field) to achieve a targeted abundance of product ions.
Against this background, there is a need in the mass spectrometry art for a method and apparatus for performing CID in a QIT with improved dissociation efficiency, thereby enhancing instrument sensitivity and/or throughput.